Most huntingtin-lowering treatments in development or clinical trials are not selective for the mutant allele, but also lower wildtype huntingtin expression. The purpose of this project is to fill critical gaps in our knowledge about the effects of wildtype huntingtin lowering in the primate brain over an extended period of time in brain regions relevant to Huntington?s disease (HD) and to the functions of wildtype huntingtin. Data from mouse models of HD indicate that lowering huntingtin in cortical neurons that are part of the corticostriatal system may be necessary for optimal therapeutic efficacy.1-3 Although prior research has established that lowering wildtype huntingtin by 45% in the primate striatum is well-tolerated for six months,4 this and other research5 has not lowered wildtype huntingtin in primate corticostriatal neurons whose axons comprise the corticostriatal tract. Huntingtin is involved in the rate of production6 and transport7 of BDNF (brain-derived neurotrophic factor) from cortical neurons to the striatum, on which striatal neurons depend.8 It is unknown whether reduction of wildtype huntingtin in corticostriatal neurons/tract of the primate brain will adversely affect the survival and functioning of striatal neurons. AIM 1 will assess the long-term tolerability of wildtype huntingtin lowering in the corticostriatal tract over 9 months by quantitatively measuring the effects of this lowering on locomotor activity, BDNF levels and neurotransmitter systems. As research tools, the project uses an shRNA already known to lower huntingtin in the primate4 and a serotype of AAV already known to retrogradely transduce cortical neurons from a striatal point of infusion.9 Treatment of the thalamus has been proposed as a means of enhancing vector distribution in the brain16. However, pathologic calcification of the thalamus has been reported in mice 9 months after partial, conditional knockout of wildtype huntingtin.10 AIM 2 will determine if reduction of wildtype huntingtin in the primate thalamus will result in an increased thalamic calcium accumulation, whether occurring incidentally from striatal infusion of vectors or directly by vector infusion targeting the thalamus. In the proposed studies, thirty- two (32) adult rhesus monkeys will be equally divided into four experimental groups (N= 8 animals/group) and randomly assigned to receive MRI-guided injections of AAV6 encoding a huntingtin-lowering shRNA or a control shRNA, directly into either the striatum (AIM 1) or thalamus (AIM 2). Monkeys will be assessed by periodic behavioral and neurological exams. In vivo glutamate signaling in the cortex, striatum and thalamus will be assessed 9 months after AAV administration. Post-mortem analysis will include assessments of dopamine turnover and BDNF levels in striatal tissue as well as calcium in the thalamus and neuropathological evaluations of coronal brain sections, including cortex, striatum and thalamus. The data obtained by this project in the nonhuman primate will provide information that cannot be readily obtained from human clinical trials of huntingtin-lowering treatments. This information will be pertinent to any current and future therapies for HD that use non-allele-specific means to lower huntingtin irrespective of the agent or route of delivery.